Formation coring is a well-known process in the oil and gas industry. In conventional coring operations, a core barrel assembly is used to cut a cylindrical core from the subterranean formation and to transport the core to the surface for analysis. Analysis of the core can reveal invaluable data concerning subsurface geological formations—including parameters such as permeability, porosity, and fluid saturation—that are useful in the exploration for and production of petroleum, natural gas, and minerals. Such data may also be useful for construction site evaluation and in quarrying operations.
A conventional core barrel assembly typically includes an outer barrel having, at a bottom end, a core bit adapted to cut the cylindrical core and to receive the core in a central opening, or throat. The opposing end of the outer barrel is attached to the end of a drill string, which conventionally comprises a plurality of tubular sections that extends to the surface. Located within, and releasably attached to, the outer barrel is an inner barrel assembly having an inner tube configured for retaining the core. The inner barrel assembly further includes a core shoe disposed at one end of the inner tube adjacent the throat of the core bit. The core shoe is configured to receive the core as it enters the throat and to guide the core into the inner tube. Both the inner tube and core shoe are suspended within the outer barrel with structure permitting the core bit and outer barrel to rotate freely with respect to the inner tube and core shoe, which remain rotationally stationary. Thus, as the core is cut—by application of weight to the core bit through the outer barrel and drill string in conjunction with rotation of these components—the core will traverse the throat of the core bit to eventually reach the rotationally stationary core shoe, which accepts the core and guides it into the inner tube assembly where the core is retained until transported to the surface for examination.
Conventional core bits are generally comprised of a bit body having a face surface on a bottom end. The opposing end of the core bit is configured, as by threads, for connection to the outer barrel. Located at the center of the face surface is the throat, which extends into a hollow cylindrical cavity formed in the bit body. The face surface includes a plurality of cutters arranged in a selected pattern. The pattern of cutters includes at least one outside gage cutter disposed near the periphery of the face surface that determines the diameter of the borehole drilled in the formation. The pattern of cutters also includes at least one inside gage cutter disposed near the throat that determines the outside diameter of the core being cut.
During coring operations, a drilling fluid is usually circulated through the core barrel assembly to lubricate and cool the plurality of cutters disposed on the face surface of the core bit and to remove formation cuttings from the bit face surface to be transported upwardly to the surface through the annulus defined between the drill string and the wall of the well bore. A typical drilling fluid, also termed drilling “mud,” may be a hydrocarbon or water base in which fine-grained mineral matter is suspended. The core bit includes one or more ports or nozzles positioned to deliver drilling fluid to the face surface. Generally, a port includes a port outlet, or “face discharge outlet,” which may optionally comprise a nozzle, at the face surface in fluid communication with a face discharge channel. The face discharge channel extends through the bit body and terminates at a face discharge channel inlet. Each face discharge channel inlet is in fluid communication with an upper annular region formed between the bit body and the inner tube and core shoe. Drilling fluid received from the drill string under pressure is circulated into the upper annular region to the face discharge channel inlet of each face discharge channel to draw drilling fluid from the upper annular region. Drilling fluid then flows through each face discharge channel and discharges at its associated face discharge port to lubricate and cool the plurality of cutters on the face surface and to remove formation cuttings as noted above.
In conventional core barrel assemblies, a narrow annulus exists in the region between the inside diameter of the bit body and the outside diameter of the core shoe. The narrow annulus is essentially an extension of the upper annular region and, accordingly, the narrow annulus is in fluid communication with the upper annular region. Thus, in addition to flowing into the face discharge channel inlets, the pressurized drilling fluid circulating into the upper annular region also flows into the narrow annulus between the bit body and core shoe, also referred to as a “throat discharge channel.” The location at which drilling fluid bypasses the face discharge channel inlets and continues into the throat discharge channel is commonly referred to as the “flow split.” The throat discharge channel terminates at the entrance to the core shoe proximate the face of the core bit and any drilling fluid flowing within its boundaries is exhausted proximate the throat of the core bit. As a result, drilling fluid flowing from the throat discharge channel will contact the exterior surface of the core being cut as the core traverses the throat and enters the core shoe.
Prior art core barrel assemblies are prone to damage core samples in various ways during operation. For example, a significant length of the core shoe may extend longitudinally below a core catcher housed within the core shoe. After the core catcher engages the core, withdrawal of the core barrel assembly from the well bore often causes the core to fracture at a location just below the core catcher instead of at the bottom of the well bore, leaving a stump of core material within the well bore. This stump may be problematic for several reasons. For example, this stump may dislocate the core catcher, cause the core barrel assembly to jam, or otherwise interfere with a smooth withdrawal of the core sample from the well bore. Moreover, the stump represents a portion of the core sample that was not recovered and delivered to the surface, resulting in a potential loss of valuable information regarding the formation material within the well bore. Additionally, the stump may interfere with subsequent operations within the well bore, such as drilling, reaming, or additional coring operations.
Another way in which prior art core barrel assemblies damage core samples is by exposing the core to deleterious amounts of drilling fluid. For example, a throat discharge channel having a high Total Flow Area (“TFA”), measured in a plane transverse to a longitudinal axis of the core barrel assembly, can create significant problems during coring operations, especially when coring in relatively soft to medium hard formations, or in unconsolidated formations. Drilling fluids discharged from the throat discharge channel enter an unprotected interval where no structure stands between such drilling fluids and the outer surface of the core as the core traverses the throat and enters the core shoe. Such drilling fluid can invade and contaminate the core itself. For soft or unconsolidated formations, these drilling fluids invading the core may wash away, or otherwise severely disturb, the material of the core. The core may be so badly damaged by the drilling fluid invasion that standard tests for permeability, porosity, and other characteristics produce unreliable results, or cannot be performed at all. The severity of the negative impact of the drilling fluid on the core increases with the velocity of the drilling fluid in the unprotected interval. Fluid invasion of unconsolidated or fragmented cores is a matter of great concern in the petroleum industry as many hydrocarbon-producing formations, such as sand and limestone, are of the unconsolidated type. For harder formations, drilling fluid coming into contact with the core may still penetrate the core, contaminating the core and making it difficult to obtain reliable test data. Thus, limiting fluid invasion of the core can greatly improve core quality and recoverability while yielding a more reliable characterization of the drilled formation.
The problems associated with stump length and fluid invasion of core samples described above may be a result, at least in part, of the material comprising the bit body of a core barrel assembly. Conventional core bits often comprise hard particulate materials (e.g., tungsten carbide) dispersed in a metal matrix (commonly referred to as “metal matrix bits”). Metal matrix bits have a highly robust design and construction necessitated by the severe mechanical and chemical environments in which the core bit must operate. However, the dimensional tolerances of metal matrix core bits (including inner surface diameter, gap width of the throat discharge channel, TFA of the face discharge channels and depth of the junk slots) are severely limited by the strength of the metal matrix material. In such metal matrix core bits, portions of the bit body must exceed a minimal thickness necessary to maintain structural integrity and inhibit the formation of cracks or microfractures therein.